arch: arm: minor style fixes in funtion and headers' documentation

Replace Cortex-M3 with Cortex-M architecture family
in the header documentation of kernel_arch_data.h and
kernel_arch_func.h, which are generic header files for
the entire familty of ARM Cortex-M CPUs. The commit
adds some more minor style fixes in functions'
documentation.

Signed-off-by: Ioannis Glaropoulos <Ioannis.Glaropoulos@nordicsemi.no>